The term xe2x80x9cbanknotexe2x80x9d is used herein for convenience and for ease of comprehension. However, it is to be interpreted as including any sheet-like objects having detectable features, for example tickets and vouchers, and fraudulent and counterfeit versions thereof.
It is known that magnetic signatures are printed on many types of banknote and that these signatures are consistent between banknotes of the same type. This property has been used by many manufacturers of banknote validators, in conjunction with optical methods, to determine the value of a banknote and to determine its authenticity.
Several sensor designs have be en used to detect this signature, all of which have disadvantages. A simple type uses an inductive device, similar to those found in tape recorders. These devices are only suitable for use where the banknotes to be validated produce a strong magnetic field. Also, the output of the sensor is dependant on the speed of the banknote. Magneto-resistors have been used in various configurations and have proved not to be sensitive enough.
A derivative of the magneto-resistor is the giant magneto-resistor. These devices are extremely sensitive to small magnetic fields. They are so sensitive that they can detect ferrous materials at considerable distances, making the use of these devices in an unshielded plastic casing impractical. Furthermore, the range of fields that can be measured is very limited and fields from motors and power transformers easily overwhelm the field from a banknote. There are devices that address these problems. However the cost of these devices makes them unsuitable for use in a low cost banknote validator.
According to the present invention, there is provided a magnetic sensor comprising a magnetic circuit and an electronic circuit, the magnetic circuit comprising a yoke and a giant magneto-resistor and the electronic circuit comprising a coil arranged to generate a magnetic field in the yoke and a feedback control loop responsive to the output of the giant magneto-resistor to energise the coil so that the giant magneto-resistor operates in a predetermined region of its characteristic.
Preferably, the frequency response of the control system has a low-pass characteristic. Thus, the bias field applied to the giant magneto-resistor compensates for stationary and relatively slowly changing ambient magnetic fields. In the particular case of a magnetic sensor for a banknote validator, it has been found that a low-pass characteristic with a first order roll-off with a xe2x88x923 dB point in the range 1 to 5 Hz is desirable. Preferably, however, the xe2x88x923 dB point is at 2 Hz.
While large stationary or slowly changing ambient magnetic fields can be handled by feedback control of the giant magneto-resistor""s magnetic bias, there remains the problem of more rapidly changing magnetic fields.
According to the present invention, there is provided a magnetic sensor comprising two giant magneto-resistors connected by a yoke, and a subtracter configured for subtracting the output of one of the giant magneto-resistors from that of the other, wherein the giant magneto-resistors are arranged such that only one of the giant magneto-resistors is significantly sensitive to magnetic fields generated in a sensing region and both giant magneto-resistors are sensitive to ambient magnetic fields. Consequently, the components of the giant magneto-resistor outputs due to ambient fields cancel and the output from the subtracter is substantially only dependent on the local field detected substantially by only one of the giant magneto-resistors.
The characteristics of the giant magneto-resistors need to be matched. This can be ensured by carefully selecting the giant magneto-resistors to be used together. A preferred alternative is to employ first bias means for applying a constant bias voltage to one of the giant magneto-resistors and second bias means for applying a variable bias voltage to the other giant magneto-resistor, the second bias means being responsive to the output of the subtracter to generate a bias voltage tending to cause the output of the subtracter to be zero. The closed-loop transfer function of the second bias means should be arranged such that desired signals are not significantly attenuated.
Preferably, the yoke comprises two connected arms, one giant magneto-resistor is mounted between free ends of the arms of the yoke, and the other giant magneto-resistor is mounted between the arms of the yoke between their interconnection and said one giant magneto-resistor.
The two techniques for dealing with interfering magnetic fields set out above are preferably combined.
It will be appreciated that applications of magnetic sensors according to the present invention extend far beyond the particular case of sensing magnetic characteristics of banknotes. For instance, such sensors could be used for sensing magnetic characteristics of coins or for reading magnetic recordings.
There are many methods of obtaining a characteristic waveform from a banknote using optical techniques. Typically, a banknote to be validated is illuminated with narrowband light and the amplitude of light reflected and/or transmitted by a banknote measured.
According to the present invention, there is provided a banknote validator including an optical sensor for sensing optical characteristics of a banknote being validated, the sensor comprising a light source, incident light-directing means for directing light from the light source onto a banknote being validated, a photodetector and reflected light-directing means for directing light from the light source, after reflection from a banknote being validated, to the photodetector, characterized in that the light source is a source of broadband light and an optical filter is interposed between reflected light-directing means and the photodetector.
This arrangement takes advantage of all of the light wavelengths that the banknote can reflectively filter. As a result, more distinctive information is yielded. Suitable broadband sources include incandescent bulbs of various types and also broadband light emitting diodes which produce light across substantially the whole of the visible spectrum. The filter responses of the receivers are such that the banknote""s properties can be sorted into selected areas of activity to match the banknote designer""s chosen wavelength response. When using a narrowband source, a truly distinctive characteristic is only obtained if the wavelength, produced by the narrowband source, is part of the filtering effect of the banknote.
Preferably, a light guide serves as the incident light-directing means and the reflected light-directing means. Conveniently, the light guide is a substantially trapezial, planar solid, the narrow end of which is adjacent the light source and the photodetector and the broad end of which is adjacent a banknote path.
Preferably, the optical sensor comprises a plurality of photodetectors and a plurality of optical filters to which light is directed by the reflected light-directing means, the optical filters having different transmission characteristics and being associated with respective photodetectors.
The filter may be one that passes primarily infrared light or blue-green light. Infrared and blue-green light-passing filters may be arranged in series. Filters having the following 3 dB stopbands have been found to be preferable: 420-720 nm and 480-540 nm together with  greater than 820 nm. The filters may be arranged in series.
When reflecting from a specular surface the power of light reflected back in a particular direction is proportional to the degree of specularity and the diffuse behaviour of the surface. Banknotes contain both specular and diffuse surfaces as part of their design, the main surface being predominantly diffuse. Areas of specular reflection are created by using highly reflective devices such as flechetes, plastic holograms, and metalised threads.
The present inventors have discovered that directing light obliquely onto a banknote helps to create highly distinctive waveforms when scanning banknotes using an opto-reflective technique.
According to the present invention, there is provided a banknote validator including an optical banknote sensor configured to sense light reflected by a banknote being validated, characterized in that the sensor is configured to sense light reflected obliquely from a banknote being validated.
Preferably, the sensor is configured to sense light reflected from a banknote being validated at an angle in the range 60xc2x0 to 80xc2x0 to the surface of the banknote at the point of reflection. 70xc2x0 has been found to be the optimum angle.
Preferably, the optical banknote sensor comprises a light guide for guiding light from a banknote being validated to a photodetector. More preferably, the light guide comprises a transparent, trapezial, planar solid having a narrow end and a broad end, the narrow end being adjacent the photodetector and the broad end being adjacent a banknote path. The internal angles between the main faces of the light guide and the broad end face are preferably 70xc2x0 and 110xc2x0 respectively.
The same light guide may be used for directing sensing light from a light source onto a banknote being validated.
According to the present invention, there is provided a banknote validator comprising a banknote path, a non-return gate in the banknote path, reversible banknote driving means for driving a banknote in the banknote path, banknote characteristic sensing means and processing means operable to operate the banknote driving means in a first direction during sensing of banknote characteristics by the banknote characteristic sensing means and thereafter reverse the banknote driving means to reject or accept a banknote, wherein the processing means is responsive to the output of the banknote characteristic sensing means to identify an acceptable banknote and, if a banknote is identified as being acceptable, to reverse the banknote driving means only after the banknote has cleared the non-return gate. Such a banknote validator has the advantage of simplified control of the banknote driving means. The difference between a banknote being accepted and a banknote being rejected is the timing of the reversing of the banknote driving means.
Preferably, the non-return gate includes banknote-guiding means arranged for guiding an acceptable banknote along a banknote accept path when the banknote driving means is reversed. The banknote-guiding means may comprise a surface of a plurality of surfaces, arranged side-by-side. The banknote-guiding means is preferably curved in the direction of banknote travel. The smaller angle between the banknote guiding means and an acceptable banknote should be no more that 50xc2x0 when the leading edge of the banknote contacts the banknote guiding means. If this angle is larger, the banknote is liable to crumple, jamming the validator.
Preferably, the non-return gate comprises pivotably mounted flap means biased into the banknote path and extending in the direction of travel of a banknote before reversal of the banknote driving means. More preferably, the flap means is pivoted into a open position by contact with a banknote passing in a banknote insertion direction along the banknote path. This has the advantage of avoiding the need for an actuator for opening and closing the non-return gate.
A preferred embodiment includes a rotatable banknote guide located behind the non-return gate and a banknote guide wall, and the banknote driving means includes a banknote driving wheel below the rotatable banknote guide, and an acceptable banknote is guided by the non-return gate and the banknote guide wall up and rearwardly over the rotatable banknote guide when the banknote driving means is reversed.
Preferably, the non-return gate extends substantially completely across the width of the banknote path.
Preferably, the underside of the flap means has a projection and the banknote path has a depression, the projection being received in the depression when the flap means is in its banknote path blocking position. There may be a plurality of such projections and depressions, for instance ribs on the flap means and grooves in the floor of the banknote path.
The various aspects of the present invention set out above may be embodied singly or in any combination in a banknote validator.